Journal of Saudi Chemical Society (2016) 20, 553–560

King Saud University

Journal of Saudi Chemical Society

www.ksu.edu.sawww.sciencedirect.com

ORIGINAL ARTICLE

Physical properties of Fe doped Mn3O4 thin films

synthesized by SILAR method and their

antibacterial performance against E. coli

* Corresponding author at: Nanostructured Thin Film Materials

Laboratory, Department of Physics, Govt. V.I.S.H., Amravati 444604,

Maharashtra, India. Tel.: +91 721 2531706; fax: +91 721 2531705.

E-mail address: ashokuu@yahoo.com (A.U. Ubale).

Peer review under responsibility of King Saud University.

Production and hosting by Elsevier

http://dx.doi.org/10.1016/j.jscs.2014.11.0041319-6103 ª 2014 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

M.R. Belkhedkara,b, A.U. Ubale

a,*

a Nanostructured Thin Film Materials Laboratory, Department of Physics, Govt. Vidarbha Institute of Scienceand Humanities, VMV Road, Amravati 444604, Maharashtra, Indiab Department of Physics, Shri Shivaji College, Akola 444003, Maharashtra, India

Received 20 September 2014; revised 20 November 2014; accepted 24 November 2014Available online 10 December 2014

KEYWORDS

Thin films;

Chemical synthesis;

Optical properties;

Electrical properties;

Antibacterial activity

Abstract Nanocrystalline Fe doped Mn3O4 thin films were deposited by successive ionic layer

adsorption and reaction method onto glass substrates. The X-ray diffraction study revealed that

Fe doped Mn3O4 films are nanocrystalline in nature. The morphological investigations were carried

out by using field emission scanning electron and atomic force microscopy studies. The optical

absorption measurements showed that Mn3O4 films exhibit direct band gap energy of the order

of 2.78 eV and it increased to 2.89 eV as the percentage of Fe doping in it increases from 0 to

8 wt.%. The room temperature electrical resistivity of Mn3O4 increases from 1.84 · 103 to

2.64 · 104 � cm as Fe doping increases from 0 to 8 wt.%. The SILAR grown Mn3O4 showed anti-

bacterial performance against Escherichia coli bacteria which improved remarkably with doping.ª 2014 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is

an open access article under theCCBY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

1. Introduction

For the last twenty years, nanocrystalline thin films of transi-

tion-metal oxides have been extensively studied owing to theirfascinating novel physical and chemical properties. The

fabrication of nanocrystalline thin films in unlimited quantitieswith outstanding fundamental and potential technological

consequences is the crucial challenge of modern materialsresearch. At nano size, semiconductor crystallite due to quan-tum confinement of the electronic states shows different phys-

ical and chemical properties in comparison with bulk crystals.These ultra-thin semiconductor thin films have potential appli-cations in various types of optoelectronic devices. Severalefforts have been made by researchers to synthesize thin metal

oxide semiconductor thin films by a simple and economicchemical method [1–5]. In the present work an attempt wasmade to deposit nanocrystalline Fe doped Mn3O4 thin films

by the simple chemical successive ionic layer adsorption andreaction method. It is well known that manganese oxide hasdifferent phases such as MnO, MnO2, Mn2O3, and Mn3O4

554 M.R. Belkhedkar, A.U. Ubale

depending on oxidation states of Mn. Among them Mn3O4 is apromising material which is useful in various types of applica-tions such as catalysts [6], rechargeable lithium batteries [7],

electrochemical materials [8], electrochemical sensors [9], sup-ercapacitors [10] etc. In addition, this utmost stable oxidephase of manganese has been used as an absorbing material

in various types of optoelectronic devices [11]. As per the liter-ature many researchers have prepared manganese oxide thinfilms by doping suitable metal ions to modify their structural,

morphological, optical and electrical properties. Bayon et al.[12] have studied the optical properties of dip coated copperdoped manganese oxide thin films. Hussain et al. [13] haveinvestigated structural and electrical properties of lithium

doped manganese oxide thin films grown by pulsed laser depo-sition. However, Singh et al. [14] have reported microstructuraland electrochemical properties of lithium doped manganese

oxide thin films grown by pulsed laser deposition. Dakhel[15] has reported dc conduction mechanisms in dysprosiumdoped manganese oxide thin films grown on Si substrates by

the thermal deposition method. Yang [16] has studied superca-pacitor application of pulsed laser deposited cobalt-dopedmanganese oxide thin films. Moon et al. [17] have synthesized

Li doped manganese oxide thin films by R.F. magnetronsputtering and investigated the effect of film stress on itsmicrostructure, surface morphology, and electrochemical char-acteristics. However, as per our knowledge no report is avail-

able on structural and electrical properties of Fe doped Mn3O4

thin films grown by the simple and economic chemical method.In the present work, the simple and economic chemical

method, successive ionic layer adsorption and reaction methodhas been successfully employed to deposit Fe doped Mn3O4

thin films at room temperature. The structural, morphological,

optical and electrical characterizations of Fe doped Mn3O4

thin films were investigated. In addition, the antibacterial per-formance of the films against Escherichia coli bacteria is

discussed.

2. Experimental

2.1. Preparation Fe doped Mn3O4 thin films

In the SILAR method, to deposit nanocrystalline thin films,

substrate is alternately immersed into cationic and anionic pre-cursors repeatedly. After each immersion, the substrate isrinsed in deionized water to remove loosely bound species

from it. To deposit Fe doped Mn3O4 thin films, 0.06 M FeCl3of pH <1 and 0.3 M MnCl2 of pH 12 were used as cationicprecursors along with dilute NaOH (pH � 12) solution as an

anionic precursor. Several trials were carried out to optimizethe various deposition parameters for a-Fe2O3 and Mn3O4

thin films separately. The optimized deposition parameters

for Mn3O4 films were already explained elsewhere [18]. Todeposit a-Fe2O3 thin films, a well cleaned glass substrate wasimmersed in cationic precursor for 20 s where Fe3þ. Ions wereadsorbed on the substrate surface. The substrate was then

rinsed in deionized water for 20 s to remove loosely boundions. Finally, it was immersed in NaOH solution for 20 s,where OH� ions react with Fe3þ ions to form a-Fe2O3 species.

This was followed by rinsing in deionized water for 20 s toremove loose a-Fe2O3 species from the substrate surface. Thiscompletes one SILAR deposition cycle for a-Fe2O3. Several

deposition trials were performed by varying the concentrationof Fe source to match the growth rate of a-Fe2O3 film forma-tion with Mn3O4. It was observed that the growth rate of

Mn3O4 and a-Fe2O3 at 0.06 and 0.3 M concentration of FeCl3and MnCl2 was approximately same. Hence, for the presentwork, to deposit Fe doped Mn3O4 thin films 0.06 and 0.3 M

concentrations of FeCl3 and MnCl2 were considered. Toachieve 0, 2, 4, 6 and 8 wt.% doping of Fe in Mn3O4 theSILAR deposition cycles for Fe2O3: Mn3O4 composition were

taken as (0:50), (1:49), (2:48), (3:47) and (4:46) respectively.

2.2. Characterization techniques

In the present work, thickness of the film was measured by thegravimetric weight difference method using relation,

t ¼ m

q� Að1Þ

.where ‘m’ is the mass of the deposited film measured by sen-sitive microbalance; ‘A’ is the area of the deposited film and

‘q’ is the density of the deposited materials in bulk form.The crystal structure of the deposited film was identified bygrazing incidence X-ray diffraction Xpert PRO PANalytical

diffractometer. The film surface morphology was observedby using field emission scanning electron microscope (Model:SUPRA 40) and atomic force microscope (Model: Nanonics

Multiview 2000�, Israel). The optical absorption studies werecarried out in the wavelength range 350–750 nm using ELICO� Double Beam SL 210 UV–VIS spectrophotometer. The var-iation of electrical resistivity with temperature was measured

using the dc two point probe technique with a digital electrom-eter and stabilized power supply.

2.3. Antibacterial test

The antibacterial performance of Fe doped Mn3O4 thin filmsagainst E. coli was investigated using the spread plate tech-nique. Initially, the culture of E. coli bacteria was prepared

in nutrient broth. The loopful culture of E. coli organismswas then inoculated into 20 mL sterilized nutrient broth andincubated at 310 K temperature for 24 h to obtain well grownbacteria. Then, 20 lL culture of E. coli was inoculated on Fe

doped Mn3O4 deposited and undeposited glass substrates ofarea 1 cm2 with the help of inoculating needle. These glassslides were then placed in previously sterilized petri dishes

and incubated at 310 K for 24 h. After the successful incuba-tion these slides were then washed by ultrasonication using3 mL buffer peptone solution to detach bacteria from the sub-

strate. Further, 20 lL washed buffer peptone solution was theninoculated on nutrient agar plates by the spread plate tech-nique and incubated at 310 K for 24 h to obtain viable bacte-

ria. After successful incubation the viable bacterial colonieswere counted and antibacterial efficiency was calculated usingthe relation [19],

r ¼ ðN0 �NÞN0

� 100% ð2Þ

where, ‘r’ is the antibacterial efficiency, ‘N0’ is the number ofviable bacteria with undeposited (standard) sample in the petri

dish and ‘N’ is the number of viable bacteria with depositedsample in the petri dish.

20

180

340

#(1

1 1)

*(1

1 3)

*(0

2 4)

#(1

3 4)

#(1

5 3)

#Mn3O4: JCPDS - 75-0765* Fe2O3: JCPDS - 79-0007*(

0 1

2)

#(1

1 2)

#(2

0 0)

50

150

250

#

#

# # #

*

*

0

120

240

Inte

nsity

(co

unt)

#

#

# # #

*

0

100

200

#

#

# # #

0

45

90

20 40 60 802θ (degree)

#

#

# # #

Figure 1 GIXRD patterns of Fe doped Mn3O4 thin films.

Table 1 Crystallite size of Fe doped Mn3O4 thin films with

doping percentage.

Wt.% of

Fe in Mn3O4

Film

thickness (nm)

Average crystallite size from

GIXRD (nm) FESEM (nm)

0 104.6 43 46

2 105 38 41

4 106 32 35

Physical properties of Fe doped Mn3O4 thin films 555

3. Results and discussion

3.1. Film formation mechanism

The SILAR deposition mechanism involves the ion-by-iondeposition at nucleation sites on the substrate surfaces. It is

well known that, in aqueous solution MnCl2, FeCl3 andNaOH dissociates as,

MnCl2 $Mn2þ þ 2Cl� ð3Þ

FeCl3 $ Fe3þ þ 3Cl� ð4Þ

NaOH$ Naþ þOH� ð5Þ

Initially, the substrate was immersed in MnCl2 precursor,where Mn2+ ions get adsorbed on the substrate. After rinsing

in deionized water, the substrate was immersed in an alkalinemedium i.e. in NaOH precursor, where 2OH� is from thesolution reacts with Mn2+ on the substrate surface to give

Mn(OH)2,

Mn2þ þ 2OH� ! nðOHÞ2 ð6Þ

To remove loose species of Mn(OH)2, the substrate wasremoved from NaOH precursor and immersed in rinsingwater. During this process Mn(OH)2 was transformed toMn3O4 by oxidation with atmospheric oxygen [18],

6MnðOHÞ2 þO2 ! 2Mn3O4 þ 6H2O ð7Þ

Finally, the Mn3O4 coated glass substrate was immersed inFeCl3 precursor, where Fe3þ ions get adsorbed on the sub-strate to give Fe3þMn3O4

Mn3O4 þ FeCl3 ! Fe3þMn3O4 ð8Þ

Further, when the substrate was immersed in NaOH pre-cursor solution, adsorption of hydroxide ions on the substratetakes place as,

Fe3þMn3O4 þ 3OH� ! FeðOHÞ3Mn3O4 ð9Þ

After annealing the film in air atmosphere at 573 K com-posite a-Fe2O3: Mn3O4 material is formed as,

2FeðOHÞ3Mn3O4

573K

3h! 2a� Fe2O3 : Mn3O4 ð10Þ

6 108 30 32

8 107 29 30

3.2. Structural studies

Fig. 1 shows GIXRD patterns of Fe doped manganese oxide

thin films deposited onto glass substrates by the SILARmethod. The (111), (112), (134), (2 00) and (153) peaksobserved in the X-ray diffraction patterns corresponds to theorthogonal structure of Mn3O4 [JCPDS 75-0765]. However

(012), (113) and (024) peaks correspond to the rhombohedralstructure of a-Fe2O3 [JCDPS 79-0007]. It is observed that theintensity of the diffraction peaks is decreased with the percent-

age of Fe doping in Mn3O4 films. The average crystallite size ofthe film was determined by using the Scherrer formula [20],

D ¼ 0:9kb Cosh

ð11Þ

where ‘k’ is the wavelength used (0.154 nm); ‘b’ is the angularline width at half maximum intensity in radians; ‘h’ is the

Bragg’s angle. It is seen that the average crystallite size ofthe as deposited Mn3O4 is 43 nm and it decreases to 29 nmas Fe doping increases to 8 wt.%. (Table 1)

3.3. Surface morphology

The surface morphology of the Fe doped manganese oxide

thin films deposited onto glass substrates by the SILARmethod by changing Fe doping from 0 to 8 wt.% was exam-ined using FESEM images (Fig. 2). The images showed

uniform distribution of spherical nanograins of Mn3O4 anda-Fe2O3 over the entire substrate surface. The size of the nano-grains varies between 27 to 49 nm depending on the dopingpercentage and it agrees well with XRD results (Table 1). Also

A B

C D

E F

Figure 2 FESEM images of Fe doped Mn3O4 thin films with Fe doping percentage: (A) 0 wt.%, (B) 2 wt.%, (C) 4 wt.%, (D) 6 wt.% and

(E) 8 wt.%, and (F) typical EDX spectrum of 2 wt.% Fe doped Mn3O4 thin film.

556 M.R. Belkhedkar, A.U. Ubale

as the doping percentage of Fe increases above 4 wt.% theagglomeration of nanograins is observed in terms of over-growth on the background of the homogeneous granular struc-

ture. A typical EDX spectrum shown in Fig. 2(F) confirmsdoping of Fe element in the Mn3O4 film material. However,the other peaks in the EDX pattern are due to the glass

substrate.The surface morphology of Fe doped Mn3O4 films were

further investigated by using atomic force microscope intapped mode. Fig. 3 shows 3D AFM images of Fe doped

Mn3O4 thin films. The films are uniform, homogeneous andwell covered to the glass substrates. The rms roughnessof Mn3O4 film decreases as Fe doping in it increases from

0–4 wt.% owing to its improved nanocrystalline nature.Above 4 wt.% doping of Fe rms roughness of the filmsincreases due to overgrowth as seen in FESEM images. The

other morphological parameters estimated from AFM analy-sis are tabulated in Table 2. The average height estimated isalso in good agreement with films thickness estimated usingthe gravimetric method.

(A) (B)

(C) (D)

(E)

Figure 3 3D AFM images of Fe doped Mn3O4 thin films with doping percentage (A) 0 wt.%, (B) 2 wt.%, (C) 4 wt.%, (D) 6 wt.% and

(E) 8 wt.%.

Physical properties of Fe doped Mn3O4 thin films 557

3.4. Optical studies

The optical properties of undoped and Fe doped Mn3O4 thinfilms deposited onto glass substrates were studied in the wave-length range 350–750 nm at room temperature (Fig. 4). It was

observed that the optical absorption of Mn3O4 decreases asdoping percentage of Fe increases. The optical absorption

coefficient ‘a’ for Mn3O4 at 350 nm wavelength is of the orderof 0.1805 cm�1 and it decreases to 0.0516 cm�1 as the dopingof Fe rises to 8 wt.%, indicating that Fe doping increases the

tα

t

0

0.05

0.1

0.15

0.2

350

2%4%6%8%

450

undop Fe Fe Fe Fe

ed Mn doped doped doped doped

3O4 Mn Mn Mn Mn

Wavel

3O43O43O43O4

550

ength, λλ (nm)

650 750

Figure 4 Plots of optical absorption against wavelength (k)(Inset shows the plots of (ahm)2 versus hm) for Fe doped Mn3O4

thin films.

Table 3 Variation of optical band gap and antibacterial

efficiency of Mn3O4 films with Fe doping.

Fe doping

in Mn3O4 (wt.%)

Optical band gap

energy (eV)

Activation

energy (eV)

Antibacterial

efficiency (%)

0 2.78 0.12 86

2 2.80 0.13 87

4 2.82 0.14 88

6 2.85 0.19 89

8 2.89 0.29 90

2.5

3

3.5

4

4.5

3.532.52

Log

ρ(Ω

cm)

1/T× 103 (K-1)

Undoped Mn3O4

2% Fe doped Mn3O4

4% Fe doped Mn3O4

6% Fe doped Mn3O4

8% Fe doped Mn3O4

Figure 5 Plots of logq versus 103/T for Fe doped Mn3O4 thin

films.

Table 2 Morphological parameters of Fe doped and undoped Mn3O4 thin films.

Fe doping in

Mn3O4 (wt.%)

RMS roughness

Rq (nm)

Average surface

roughness Ra(nm)

Maximum

height (nm)

Average

height (nm)

Grain

orientation (pi)

0 42.67 34.38 226.1 110.73 0.07

2 44.13 33.19 209.2 113.93 0.17

4 32.21 24.89 195.5 107.61 0.07

6 37.83 31.75 202.6 110.05 0.19

8 44.56 35.87 236.1 115.63 0.20

558 M.R. Belkhedkar, A.U. Ubale

transparency of the film. The optical band gap energy (Eg) of

undoped and Fe doped Mn3O4 films is calculated using theequation [21],

aht ¼ Aðht� EgÞn ð12Þ

where, ‘a’ is absorption coefficient, ‘Eg’ is band gap, ‘A’ is a

constant and ‘n’ is equal to 1/2 for direct and 2 for indirecttransition. The plots of (aht)2 versus ht for Fe doped Mn3O4

thin films are shown in the inset of Fig. 4. The optical band

gap energy of Mn3O4 film is 2.78 eV and it increases to2.89 eV as Fe doping increases from 0 to 8 wt.% (Table 3).This increase in the direct band gap energy of Mn3O4 films

with Fe doping is attributed to Moss-Burstein effect. Whenthe concentration of Fe doping in the Mn3O4 increases, theconduction band becomes significantly filled and the lowestenergy states in the conduction band are blocked, as a results

the absorption interband moves to higher energy states andblue shift occurs [22].

3.5. Electrical resistivity

The variation of electrical resistivity of Fe doped Mn3O4 thinfilms with temperature was studied using a dc two point probe

method. Fig. 5 shows the variation of log (q) with the recipro-cal of the temperature. The electrical resistivity of filmsdecreases with an increase in temperature that confirms their

semiconducting nature. The electrical resistivity of theMn3O4 film at 333 K temperature is of the order of1.84 · 103 � cm and it increases to 2.6 · 104 � cm as the Fedoping increases to 8 wt.%. This increased electrical resistivity

is attributed to the reduced crystallite size of the film with Fedoping percentage. As the grain size of the deposited film

decreases, the charge carrier density of the film decreases, asa result film resistivity increases. On the other hand, in theFe doped Mn3O4 film, Fe-ions act as acceptors and majority

charge carrier Mn-ions act as donors. As the Fe content inMn3O4 thin films increases, the mobility and concentrationof Mn-ions decrease that increases its electrical resistivity

[23]. The activation energies of Fe doped Mn3O4 films are cal-culated using the relation,

q ¼ qo

Ea

KT

� �ð13Þ

where ‘q’ is the resistivity at temperature T, ‘q0’ is a constant;‘K’ is the Boltzmann constant and ‘Ea’ is the activation energy.

A B

C D

FE

Figure 6 Test results on E. coli after 24 h: incubated with (A) undeposited glass substrate, and Fe doped Mn3O4 thin films: (B) 0 wt.%,

(C) 2 wt.%, (D) 4 wt.%, (E) 6 wt.% and (F) 8 wt.%.

Physical properties of Fe doped Mn3O4 thin films 559

The activation energy of Fe doped Mn3O4 thin film depositedonto glass substrate is increased from 0.12 to 0.29 eV depend-ing on doping percentage (Table 3). This increase in activation

energy is attributed to the improved nanocrystalline nature ofthe film.

3.6. Antibacterial activity

The antibacterial efficiency of the Fe doped Mn3O4 thin filmsagainst E. coli bacteria was studied. The test results of E. coli

bacteria incubated for 24 h on undeposited glass substrate andFe doped Mn3O4 thin film surfaces are shown in Fig. 6. It wasobserved that, the antibacterial efficiency of un-doped Mn3O4

film is 86% and it increases to 91% as doping of Fe increasesfrom 0 to 8 wt.% (Table 3). This increased antibacterial effi-ciency with doping is attributed to the increased contributionof Mn, Fe and hydroxyl ions released from the surface that

kills E. coli microorganisms. Also with Fe doping, the film

becomes more nanocrystalline and hence provides more sur-face area for antibacterial activity.

4. Conclusions

Nanocrystalline Fe doped Mn3O4 thin films were successfullygrown by the successive ionic layer adsorption and reactionmethod onto glass substrates. The GIXRD, FESEM andAFM analysis confirm that SILAR grown films are nanocrys-

talline in nature. The optical band gap energy, electrical resis-tivity and activation energy of Mn3O4 thin films increases withFe doping owing to improved nanocrystalline nature. The

morphological study showed that the nanocrystalline natureof the film increases with doping. The antibacterial efficiencyof Mn3O4 film against E. coli bacteria increases with Fe

doping.

560 M.R. Belkhedkar, A.U. Ubale

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